Thermoplastic resins such as polyethylene terephthalate (PET) are commonly used to manufacture packaging materials. PET processed under the right conditions produces high strength articles with excellent gas barrier properties. Foods, beverages, and medicines can deteriorate or spoil if exposed to oxygen. To improve shelf life and flavor retention of products such as foods, beverages, and medicines, therefore, the barrier protection provided by PET is often supplemented with additional layers of packaging material or with the addition of oxygen scavengers.
Adding a layer of gas barrier film is known as passive-barrier packaging. Ethylvinyl alcohol (EVOH), Polyvinylidene dichloride (PVDC), and Nylon MXD6, are examples of films commonly used for this purpose due to their excellent oxygen barrier properties. Using distinct layers of differing materials is not preferred, however, because it adds cost to packaging construction and does not reduce the levels of oxygen already present in the package.
Adding oxygen scavengers to the PET resin is known as active-barrier packaging. This approach to protecting oxygen-sensitive products is two-fold; the packaging prevents oxygen from reaching the product from the outside, and also absorbs some of the oxygen present in the container and from within the polymer matrix. In some applications, small packets or sachets containing oxygen scavengers are added to the packaging container and lie next to the food. Sachets are generally limited to solid foods, where the sachet can be readily removed from the foodstuff and not accidentally ingested. Construction of the sachets and the cumbersome nature of their introduction into the package result in increased costs.
One way to overcome the disadvantages of sachets is to incorporate the scavenger directly into the wall of the food package. This can be done by placing the scavenger throughout the scavenger wall or placing the scavenger in a unique layer between many layers of the container sidewall. It should be appreciated that references to the sidewall and wall include the lid and bottom sides of the container. At present the incorporation of the scavenger throughout the container wall is found in non-transparent trays or packaging films where the scavenger is not visible. Virtually any scavenger can be used in this application because the scavenger is not visible. However, containers requiring clarity have heretofore been limited to organic type scavengers that maintain their clarity when placed in a separate layer in the wall of the container. The use of the organic scavenger in a mono-layer or single-layer construction is limited by cost and regulatory constraints due to the nature of the organic scavenger or the by-products of the scavenging reaction.
Contributing to the cost is the logistical problems encountered with the use of organic type scavengers. In most embodiments, a transition metal catalyst is used to activate an oxidizable polymer. A disadvantage of this technique is that the polymer begins reacting with oxygen as soon as the package is made. Consequently, the bottles must be filled immediately. Higher amounts of scavenger are used to compensate for the scavenging capacity lost between the time the bottle is made and when the bottle is filled.
In another technique, UV radiation is used to activate the oxidizable polymer. However, UV activation techniques are relatively expensive, and the initiators are often not regulated for use in food packaging. Packages designed for beers and juices are specifically designed to prevent UV penetration, hence UV activation would not be practical for these containers which block UV.
An alternative to a visually acceptable organic material is to use discrete scavenging particles in the container sidewall, such as reduced metal powders. Reduced iron powder is commonly used for oxygen scavenging in food packages. Iron reacts with the oxygen and forms iron oxide. Most applications also utilize a salt and a moisture absorber as reaction-enhancing agents to increase the effectiveness of the iron powder. Because the reaction usually requires water, the iron scavenging composition remains inactive until the package is filled and the reaction is activated by the water of the packaged contents which migrates into the polymer and contacts the scavenging composition.
The use of scavenging powders in clear packages has previously been limited by aesthetics, particularly haze and color. High loadings of iron powder, on the order of 500-5000 parts per million, are typically required to obtain sufficient oxygen absorption. Conventional wisdom and prior art teaches the practitioner to use the highest amount of scavenging surface area possible so that the efficiency and capacity is increased and the amount of iron added is minimized. In practice, this means a large number of small particles. Unfortunately, previous attempts at preparing resin compositions comprising high levels of small particles of iron for use in clear packages have resulted in packages with poor optical properties. This is particularly true when the resin composition is stretched or oriented to any degree in forming the final article, such as in polyester bottles. Typically, bottles prepared from such resin compositions are translucent. Haze values for these bottles are generally high, and clarity is lacking.
Thus, there remains a need for packaging materials having acceptable visual aspects and comprising activatable oxygen scavenging resin compositions.